1. Technical Field
The present invention relates to an electrical power supply system and more particularly to an electrical power supply system for an automotive vehicle for supplying power to a high power load for a short time period by increasing an alternator output.
2. Background Art
Recently the number of a high power load of an automotive vehicle such as a blower motor or an ice-removing heater on a windshield is increasing. For the high power load a system is proposed, wherein a high power is extracted by using a high voltage charging alternator on the automotive vehicle.
For example, according to a Japanese Patent No.2864887, a 3-phase alternator is driven by an oil pressure pump and an oil pressure motor laden in a vehicle, and an output frequency as well as a voltage of the alternator is kept constant by keeping a constant engine rotation after the engine is started. Thereby, a high voltage of the alternator with a high generating efficiency could be applied to the high power load and a battery through a stepping-down voltage converter. In this configuration a rated output of the alternator regardless of an engine rotating speed is obtained from an idling zone, and the alternator with a small size and a light weight is capable of supplying power to a high power with high voltage load. The alternator is also able to supply the required power to a low voltage load through an AC/DC voltage converter. Furthermore, a layout freedom of the alternator driven by an oil pressure is not limited but even enlarged, as the AC/DC voltage converter operating with a constant output frequency of the alternator can be small in size and light in weight.
According to a Japanese laid open Patent No.6-12934, other effective method for supplying a power to the high power with high voltage load is disclosed. In the art a vehicle-laden alternator output voltage with the maximum output power changes in response to a rotating speed of the alternator. To drive the high power with high voltage load which is usually existing in the idling zone, the engine rotation is being increased from an idling speed for the alternator to reach the maximum power at a second voltage set value for supplying the power, and simultaneously an attained high voltage is reduced to a voltage equal to a first voltage set value by a voltage reducing means for charging the secondary battery.
In the former prior art although the stable alternator output is obtainable due to a constant speed driving, a rotating speed with the maximum output varies according to a load condition of the alternator. Moreover, obtaining a high voltage and a voltage for charging the secondary battery is incompatible, and an additional driving device like an oil pressure motor becomes required.
In the latter prior art simultaneous realization of the voltages corresponding respectively to the first and the second voltage set values is intended, but due to a limitation of the alternator size sufficient power is not always supplied to the high power with high voltage load during the operation at the maximum output voltage point, and a control system becomes complicated for controlling a load distribution.